Petar N. Miljanic et al in an article "An Electronically Enhanced Magnetic Core for Current Transformers" published in IEEE Transactions on Instrumentation and Measurement, Vol 40, No. 2, pp 410-414, April, 1991 (see also U.S. Pat. No. 4,841,236 issued Jun. 20, 1989) have described a technique that increases the apparent permeability of the magnetic core of a current transformer by electronically providing the magnetizing current for the core, whereby the ratio between the primary and secondary currents is rendered virtually without error.
There are two basic limitations in the various devices that have been disclosed by Miljanic et al for carrying this technique into practice.
First, while the above disclosure provided for current transformers that can operate with pure ac, or with a mixture of ac and dc, operation with dc alone was not possible.
Second, the devices were not structurally adapted for easy use with busbars. For on-site measurement of high currents in a busbar that is part of an already installed system, it is inconvenient to have to dismantle the busbar in order to thread a toroidal core onto it from one end, as is necessary if the busbar is to form a single turn primary to generate currents in a secondary winding formed on the core. This problem has been addressed in the past in some hand-held metering devices by making the transformer core in two parts that are pivoted together and can be opened to be placed over the busbar from a side thereof and then closed when in place to surround the busbar. The problem with "openable" cores of this type, however, has been the loss of accuracy that results from the increased reluctance in the core that arises from the unavoidable air gap where the two parts of the core come together. No matter how smoothly polished the mating end faces of such a core are made, the result is always a significant increase in reluctance relative to that of an uninterrupted toroidal core.